σ receptor activation blocks potassium channels and depresses neuroexcitability in rat intracardiac neurons

The sigma receptors have been implicated in the regulation of the cardiovascular system, and sigma-1 receptor transcripts have been found in parasympathetic intracardiac neurons. However, the cellular function of sigma-1 receptors in these cells remains to be determined. Effects of sigma receptor activation on voltage-activated K+ channels and action potential firing were studied in isolated intracardiac neurons using whole-cell patch-clamp recording techniques. Activation of sigma receptors reversibly blocked delayed outwardly rectifying potassium channels, large conductance Ca2+-sensitive K+ channels, and the M-current with maximal inhibition > 80%. The inhibition of K+ channels by sigma ligands was dose-dependent, and the rank order potency of (+)-pentazocine > ibogaine > 1,3-di-O-tolyguanidin (DTG) suggests that the effect is mediated by sigma-1 receptor activation. Preincubation of neurons with the irreversible sigma receptor antagonist metaphit blocked DTG-induced inhibition of K+ channels, confirming that the effect is mediated by sigma receptor activation. Although bath application of sigma ligands depolarized intracardiac neurons, the number of action potentials fired by the cells in response to depolarizing current pulses was decreased in the presence of these drugs. Neither dialysis of the neurons nor application of intracellular 5'-O-(2-thiodiphosphate) trilithium salt inhibited the effect of sigma receptors on K+ channels, which suggests that the signal transduction pathway does not involve a diffusible cytosolic second messenger or a G protein. Together, these data suggest that sigma-1 receptors are directly coupled to K+ channels in intracardiac neurons. Furthermore, activation of sigma-1 receptors depresses the excitability of intracardiac neurons and is thus likely to block parasympathetic input to the heart.